Catalyst for direct NOx decomposition and a method of forming and using the catalyst

ABSTRACT

A process of forming a direct NOx catalyst includes the steps of providing a palladium salt, providing a silicon oxide support material, mixing the palladium salt and silicon oxide support material in an aqueous solution, evaporating the aqueous solution forming a solid, calcining the solid, and then exposing the calcined solid to a pretreatment gas at a specified temperature to form a desired direct NOx catalyst. When the process includes exposing the calcined solid to helium gas at a temperature of from 650 to 1000° C. the catalyst may include a mixture of palladium and palladium oxide having a particle size of from 5 to 150 nm where the palladium particles are discrete particles without sintering and the mixture may include 41% by weight palladium oxide and 51% by weight palladium metal.

TECHNICAL FIELD

The invention relates to catalysts and with more particularity to directNOx catalysts.

BACKGROUND

More stringent NOx emission requirements for combustion engines orprocesses may require catalytic NOx abatement technologies that areeffective under various conditions. For example, catalytic NOx abatementtechnologies in order to meet higher government standards will need tobe effective under lean burning conditions. Direct NOx decomposition toenvironmentally friendly components such as nitrogen and oxygen may bean alternative to current NOx traps and selective catalytic reductioncurrently used in the prior art. The use of an effective catalyst fordirect NOx decomposition may eliminate the use of reducing agents andother complicated mechanisms to simplify the NOx removal process anddecrease a cost for NOx abatement from the exhaust of various combustionprocesses.

Typical prior art catalysts require high operating temperatures such asgreater than 700° C. to be effective for catalytic NOx abatement. Thereis therefore a need in the art for a catalyst that operates over a widetemperature range such as below 700° C. and may be utilized for directNOx decomposition. There is a further need in the art for a catalystthat may be utilized for NOx abatement without any additional secondaryreagents. There is also a need in the art for a catalyst that iseffective to decompose NOx in a temperature range of emission gas from agas or diesel engine.

SUMMARY

In one aspect, there is disclosed a process of forming a direct NOxcatalyst that includes the steps of providing a palladium salt,providing a silicon oxide support material, mixing the palladium saltand silicon oxide support material in an aqueous solution, evaporatingthe aqueous solution forming a solid, calcining the solid, and exposingthe calcined solid to helium gas at a temperature of from 650 to 1000°C. forming the direct NOx catalyst.

In another aspect there is disclosed a process of forming a direct NOxcatalyst that includes the steps of providing a palladium salt,providing a silicon oxide support material, mixing the palladium saltand silicon oxide support material in an aqueous solution, evaporatingthe aqueous solution forming a solid, calcining the solid, and exposingthe calcined solid to hydrogen gas at a temperature of from 300 to 1000°C. forming the direct NOx catalyst.

In a further aspect there is disclosed a direct NOx catalyst thatincludes a mixture of palladium, palladium oxide, and silicon oxide withthe mixture having a particle size of from 5 to 150 nm where thepalladium particles are discrete particles without sintering. Themixture may include 41% by weight palladium oxide and 51% by weightpalladium metal.

In yet another aspect there is disclosed a process of decomposing NOxthat includes the steps of providing a catalyst including a mixture ofpalladium, palladium oxide, and silicon oxide, and contacting thecatalyst with a gas at least including NOx directly decomposing the NOxto form nitrogen, nitrogen oxide, or oxygen at a temperature of from 200to 800° C.

These and additional features provided by the embodiments describedherein will be more fully understood in view of the following detaileddescription, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 is a plot of direct NOx catalysts produced by various processesincluding treatment with helium at 800° C., hydrogen at 800° C., andoxygen at 450° C.;

FIG. 2 is a plot of an x-ray photoelectron spectroscopy of the variousdirect NOx catalysts including various processes such as treatment withhydrogen at 800° C., oxygen at 450° C., and helium at 800° C.;

FIG. 3 is a plot of a photoelectron spectroscopy of catalysts treatedwith helium at a temperature of 800° C. and hydrogen at 800° C. bothafter sputtering;

FIG. 4 is a TEM image of a palladium and silicon dioxide catalyst aftertreatment in the presence of hydrogen at 800° C.;

FIG. 5 is a TEM image of a palladium, palladium oxide, and silicondioxide catalyst after treatment in the presence of helium at 800° C.;

FIG. 6 is a plot of the NOx conversion as a function of temperature forcatalysts exposed to various treatment environments including oxygen at450° C., helium at 800° C., and hydrogen at 800° C.;

FIG. 7 is a plot of the decomposition products in a NOx abatementreaction for the various catalysts exposed to different treatmentenvironments as a function of temperature.

DETAILED DESCRIPTION

In one aspect, there is disclosed a process of forming a direct NOxcatalyst that includes the steps of providing a palladium salt,providing a silicon oxide support material, mixing the palladium saltand silicon oxide support material in an aqueous solution, evaporatingthe aqueous solution forming a solid, calcining the solid, and thentreating the calcined solid with a treatment gas at a specifiedtemperature to form a desired direct NOx catalyst. Various treatmentgases and processes include exposing the calcined solid to helium gas ata temperature of from 650 to 1000° C. forming the direct NOx catalyst.Another treatment gas and process includes exposing the calcined solidto hydrogen gas at a temperature of from 300 to 1000° C. forming thedirect NOx catalyst. Another treatment gas and process includes exposingthe calcined solid to oxygen gas at a temperature of from 50-650° C.forming the direct NOx catalyst.

In one aspect, the process may utilize palladium salts such as palladiumnitrates, chlorides, or acetates. In one aspect, the calcining step mayinclude exposing the solid to a temperature of from 450° C. to 650° C.for a time period of from 3 to 10 hours.

In one aspect, when the process includes exposing the calcined solid tohelium gas at a temperature of from 650 to 1000° C. the catalyst mayinclude a mixture of palladium and palladium oxide having a particlesize of from 5 to 150 nm where the palladium particles are discreteparticles without sintering. The mixture may include 41% by weightpalladium oxide and 51% by weight palladium metal. The catalyst formedby the process may include a NOx conversion of from 20 to 40% at atemperature of from 300 to 500° C., as will be discussed in more detailbelow.

When the treatment process includes exposing the calcined solid tohydrogen gas at a temperature of from 300 to 1000° C. forming the directNOx catalyst, the catalyst may include palladium particles having aparticle size of from 50 to 2000 nm in size. The formed direct NOxcatalyst may include a NOx conversion of from 10 to 50% at a temperatureof from 300 to 800° C.

When the treatment process includes exposing the calcined solid tooxygen gas at a temperature of from 50-650° C. forming the direct NOxcatalyst, the catalyst may include palladium oxide particles. The formeddirect NOx catalyst may include a NOx conversion of from 0 to 25% at atemperature of from 650 to 800° C.

The catalyst produced by the various processes disclosed herein may beutilized to decompose NOx. The process of decomposing NOx may includeproviding a catalyst including a mixture of palladium, palladium oxide,and silicon oxide and then contacting the catalyst with a gas thatincludes at least NOx wherein NOx is directly decomposed to formnitrogen, nitrogen oxide, or oxygen at a temperature of from 200 to 800°C. The catalyst may include a mixture of palladium and palladium oxidehaving a particle size of from 5 to 150 nm where the palladium particlesare discrete particles without sintering, as described above.

EXAMPLES

Catalysts were synthesized by a wet impregnation method. 0.3247 grams ofpalladium nitrate (sigma-aldrich) was dissolved in 60 ml water and 3grams of the H₂O-treated silicon dioxide (Cabot) was added to theaqueous solution. Then the solution was heated at 80° C. until waterevaporated and was dried overnight at 120° C. Next, the solid materialwas calcined at a temperature of from 450° C. to 650° C. The calcinedsolid was then exposed to various treatment processes and conditions.The treatment processes include exposing the calcined solid to hydrogengas at a temperature of 800° C. for 30 minutes time. Additionally, thetreatment process includes exposing the calcined solid to helium gas ata temperature of 800° C. for 30 minutes time. Further, the treatmentprocess includes exposing the calcined solid to oxygen gas at atemperature of 450° C. for 30 minutes

Referring to FIG. 1, there is shown an x-ray diffraction plot of thecatalyst samples after exposure to oxygen, helium, and hydrogen asdescribed above. As can be seen in the plot, there is a cleardiffraction peak corresponding to either palladium or palladium oxide orboth materials. In one aspect, the oxygen treatment diffraction patternsdescribe a catalyst formed of 100% by weight palladium oxide. The heliumtreatment exposure results in a catalyst including 41% by weightpalladium oxide and 59% palladium metal. The hydrogen treatment processincludes a catalyst including 100% palladium metal.

Referring to FIG. 2, there are displayed XPS plots of the variouscatalysts formed by the treatments as described above. As can be seen inFIG. 2, the XPS plots show the successful synthesis of palladium andpalladium oxide on a silicon dioxide support. The plots exhibit peakscorresponding to both palladium and palladium oxide in the XPS spectra.

Referring to FIG. 3, there is shown an XPS plot for samples of thecatalysts after hydrogen and helium treatment processes as describedabove after a pre-sputtering step As can be seen in the plots, thehydrogen treated catalysts exhibit peaks due to palladium metal whilethe helium treatment catalysts exhibit peaks due to palladium metal andpalladium oxide which corroborates the formation of a catalyst includinga composite morphology that includes both palladium metal and palladiumoxide.

Referring to FIG. 4, there is shown a TEM image of the catalyst formedafter a hydrogen treatment process. As can be seen in the image, theformed direct NOx catalyst includes palladium particles having aparticle size of from 50 to 2000 nm in size.

Referring to FIG. 5, there is shown a TEM image of a direct NOx catalystformed by the helium treatment process as described above. As can beseen in the plot, the catalyst includes a mixture of palladium andpalladium oxide having a particle size of from 5 to 150 nm where thepalladium particles are discrete particles and are not sintered. NOxdecomposition experiments were performed in a vertical fixed bed flowreactor. Before reaction, the various compositions or catalysts werecreated by the treatment processes as described above. After thetreatment, the temperature of the catalysts was reduced to 100° C. andthen 1% nitrogen oxide was introduced into the reactor by ramping thetemperature from 100 to 800° C. Referring to FIG. 6, there is shown aplot of the NOx conversion efficiency for the various catalysts formedby the treatment processes. As can be seen in the plots, the catalystsexhibit a very good activity over a wide temperature range. Additionallyit can be seen that the catalysts formed by the various treatments havediffering NOx conversion percentages at different temperature ranges. Inone aspect, the direct NOx catalyst formed by exposing the calcinedsolid to a helium gas at 800° C. has a NOx conversion of from 20 to 40%at a temperature range of from 300 to 500° C. Further, the direct NOxcatalyst formed by exposing the calcined solid to hydrogen gas at atemperature of 800° C. has a NOx conversion of from 10 to 50% at atemperature of from 300 to 700° C.

Referring to FIG. 7, there is shown a plot of the selectivity of variousdecomposition components as a function of temperature for catalystsformed by the hydrogen treatment process and helium treatment process asdescribed above. As can be seen, the reaction products of nitrogen andnitrogen oxide may be produced in a temperature region of from 300 to500° C. whereas nitrogen and oxygen products may be formed attemperatures greater than 500° C. While particular embodiments have beenillustrated and described herein, it should be understood that variousother changes and modifications may be made without departing from thespirit and scope of the claimed subject matter. Moreover, althoughvarious aspects of the claimed subject matter have been describedherein, such aspects need not be utilized in combination. It istherefore intended that the appended claims cover all such changes andmodifications that are within the scope of the claimed subject matter.

What is claimed is:
 1. A process of forming a direct NOx catalystcomprising the steps of: providing a palladium salt; providing a siliconoxide support material; mixing the palladium salt and silicon oxidesupport material in an aqueous solution; evaporating the aqueoussolution forming a solid; calcining the solid; treating the calcinedsolid to helium gas at a temperature of from 650-1000° C. forming thedirect NOx catalyst.
 2. The process of claim 1 wherein the palladiumsalt is selected from the group consisting of: nitrates, chlorides andacetates.
 3. The process of claim 1 wherein the step of calciningincludes exposing the solid to a temperature of from 450° C. to 650° C.for a time period of from 3 to 10 hours.
 4. The process of claim 1wherein the formed direct NOx catalyst includes a mixture of palladiumand palladium oxide having a particle size of from 5-150 nm wherein thepalladium particles are discrete particles without sintering.
 5. Theprocess of claim 1 wherein the formed direct NOx catalyst includes 41%by weight palladium oxide and 51% by weight palladium metal.
 6. Theprocess of claim 1 wherein the formed direct NOx catalyst has a NOxconversion of from 20 to 40% at a temperature of from 300 to 500° C. 7.The process of claim 1 wherein the formed direct NOx catalyst has a NOxconversion of from 0 to 25% at a temperature of from 650 to 800° C.
 8. Aprocess of forming a direct NOx catalyst comprising the steps of:providing a palladium salt; providing a silicon oxide support material;mixing the palladium salt and silicon oxide support material in anaqueous solution; evaporating the aqueous solution forming a solid;calcining the solid; treating the calcined solid to hydrogen gas at atemperature of from 300-1000° C. forming the direct NOx catalyst.
 9. Theprocess of claim 8 wherein the palladium salt is selected from the groupconsisting of: nitrates, chlorides and acetates.
 10. The process ofclaim 8 wherein the step of calcining includes exposing the solid to atemperature of from 450° C. to 650° C. for a time period of from 3 to 10hours.
 11. The process of claim 8 wherein the formed direct NOx catalystincludes palladium particles having a particle size of from 50-2000 nm.12. The process of claim 8 wherein the formed direct NOx catalyst has aNOx conversion of from 10 to 50% at a temperature of from 300 to 800° C.13. A process of forming a direct NOx catalyst comprising the steps of:providing a palladium salt; providing a silicon oxide support material;mixing the palladium salt and silicon oxide support material in anaqueous solution; evaporating the aqueous solution forming a solid;calcining the solid; treating the calcined solid to oxygen gas at atemperature of from 50-650° C. forming the direct NOx catalyst.
 14. Theprocess of claim 13 wherein the palladium salt is selected from thegroup consisting of: nitrates, chlorides and acetates.
 15. The processof claim 13 wherein the step of calcining includes exposing the solid toa temperature of from 450° C. to 650° C. for a time period of from 3 to10 hours.